Electronic circuits for controlling a heating or air-conditioning apparatus in a motor vehicle are known, for example, from German Offenlegungsschrift No. 2,351,015 and U.S. Pat. No. 4,058,255. In such electronic circuits, the valve, which is located in a flow channel, is opened when the system for controlling the valve-activating device fails, thereby satisfying the demand of the manufacturers of motor vehicles for a capability to heat such motor vehicles even when the thermostat goes out of commission. However, significant disadvantages are connected with the known electronic circuits:
A. Every time the ignition switch is turned off--that is, every time the ignition is interrupted--the valve opens. As a result of the pumping work of the cooling agent pump, which is driven by the engine (which is slowing down), and the thermosiphon flow which is building up, hot heating water gets into the heating system's heat exchanger. If the vehicle is put back into operation before the heating water has cooled off, the heat stored in the heating system's heat exchanger goes inside the motor vehicle through the heating duct. A so-called heat surge is generated. It is especially bothersome when the ignition switch is activated briefly or frequently--for example, during short stops at coin-operated machines or letter boxes or when the engine is turned off at railroad barriers and intersections where there are traffic lights.
B. If the control of the valve is interrupted by a defect in the thermostat, the heating system is started up by the valve which then is not receiving any current and consequently is open. This can occur especially when there is high thermal stress on the thermostat's electronic equipment in warm seasons or in hot countries in general. Then the undesired turning on of the heating system is not only bothersome but it can even affect the driver of the vehicle in a way which is dangerous to traffic. Not only the driver of the vehicle but also the other electronic devices of a vehicle are affected unfavorably by too much heat inside the vehicle.
C. If, however, the system controlling the valve is short-circuited by a defect in the thermostat, the heating system is cut off by the valve, which is closed in the energized condition. This can make it impossible to use the vehicle when the outdoor temperature is low.
D. The circuit must operate continually to keep the valve closed, and it must do so even when there is no question of a need for heating the vehicle because of the ambient temperature.